Truss, permanent formwork element and slab

ABSTRACT

A truss (1) that comprises an elongate metal strip (10) and an elongate bar (12). The elongate metal strip (10) has a first longitudinally extending edge (14) and a second longitudinally extending edge (16). The first longitudinally extending edge (14) is in the form of a substantially serrated edge such that apexes (18) are formed along the first longitudinally extending edge (14). The elongate bar (12) is connected to the elongate metal strip (10) at the apexes (18) of the elongate metal strip (10) such that openings (20) are formed between the elongate bar (12) and the serrated edge of the elongate metal strip (10). A plurality of trusses (1) and one or more sheets of material (52), having trenches or channels (52) formed therein, may be used to make a permanent formwork element (50).

Throughout this specification, unless the context requires otherwise, the word “comprise” and variations such as “comprises”, “comprising” and “comprised” are to be understood to imply the presence of a stated integer or group of integers but not the exclusion of any other integer or group of integers

Throughout the specification unless the context requires otherwise, the word “include” or variations such as “includes” or “including”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

TECHNICAL FIELD

The present invention relates to a truss, a permanent formwork element incorporating a plurality of such trusses, and a slab incorporating such a permanent formwork element. The present invention also relates to a method of making such a permanent formwork element and to a method of making such a slab. The slab may be used in horizontal permanent formwork, such as flooring.

BACKGROUND

Any discussion of background art, any reference to a document and any reference to information that is known, which is contained in this specification, is provided only for the purpose of facilitating an understanding of the background art to the present invention, and is not an acknowledgement or admission that any of that material forms part of the common general knowledge in Australia or any other country as at the priority date of the application in relation to which this specification has been filed.

Various types of reinforcing bar trusses have been used for constructing concrete slabs for floors. For example, reinforcing bar trusses are typically made from metal, e.g. reinforcing steel, so as to provide reinforcing strength in precast concrete permanent formwork in floor construction. However, these precast concrete permanent formwork elements are cast into bespoke moulds depending on the shape required. These moulds are labour intensive and time consuming to set up. This results in high manufacturing costs.

SUMMARY OF INVENTION

In accordance with one aspect of the present invention, there is provided a truss comprising

an elongate metal strip having a first longitudinally extending edge and a second longitudinally extending edge, the first longitudinally extending edge being a substantially serrated edge such that apexes are formed along the first longitudinally extending edge, and

an elongate bar connected to the elongate metal strip at the apexes of the elongate metal strip,

such that openings are formed between the elongate bar and the serrated edge of the elongate metal strip.

Preferably, the elongate bar is connected to the metal strip by connecting the apexes to the elongate bar.

Preferably, the elongate bar is connected along its length to the metal strip such that the elongate bar is located substantially central to the first longitudinally extending edge of the elongate metal strip.

Preferably, the elongate bar is connected to the metal strip by welding the apexes to the elongate bar.

Preferably, the openings are substantially triangular in shape.

Preferably, the second longitudinally extending edge is rounded.

In accordance with another aspect of the present invention, there is provided a permanent formwork element comprising

a plurality of trusses, as herein before described, arranged in a substantially parallel arrangement, and

at least one sheet of material having trenches formed therein, the trenches being in a substantially parallel arrangement,

wherein the elongate metal strips are partly fixedly positioned in respective trenches such that second longitudinally extending edges of the elongate bars are positioned in the trenches.

Preferably, the elongate metal strips of the plurality of trusses are partly fixedly positioned in respective trenches such that the openings are substantially exposed above the surface of the sheet.

Preferably, the elongate metal strips of the plurality of trusses are partly fixedly positioned in respective trenches by adhesive.

Preferably, the edges of the trenches adjacent to the surface of the sheet are chamfered.

Preferably, the sheet is made of cementitious based material.

In accordance with another aspect of the present invention, there is provided a method of making a permanent formwork element incorporating a plurality of trusses as herein before described, comprising

providing at least one sheet of material having trenches formed therein, the trenches being in a substantially parallel arrangement,

inserting the second longitudinally extending edges of the elongate metal strips into respective trenches to position them in the trenches,

fixing the elongate metal strips in the trenches.

Preferably, fixing the elongate metal strips in the trenches comprises using adhesive to fix the elongate metal strips.

In accordance with another aspect of the present invention, there is provided a method of making a slab comprising calculating the expected deflections in a slab to comprise a permanent formwork element, as herein before described, and cementitious filler material to cover the permanent formwork element,

machining timber strips to have profiles that are mirror images of the profiles of the calculated expected deflections,

fixing the timber strips to supports,

positioning the permanent formwork element on the supports,

pouring cementitious material over the permanent formwork element and allowing the cementitious material to set, and

removing the supports.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a first perspective view of an embodiment of a truss in accordance with one aspect of the present invention;

FIG. 2 is a second perspective view of the truss shown in FIG. 1;

FIG. 3 is a side view of the truss shown in FIG. 1;

FIG. 4 is an end view of the truss shown in FIG. 1;

FIG. 5 is a perspective view of an embodiment of a pair of adjacent permanent formwork elements in accordance with another aspect of the present invention;

FIG. 6 is an end view of the permanent formwork elements shown in FIG. 5;

FIG. 7 is an end view of the site of one of the trusses of a permanent formwork element, of the type shown in FIG. 5, positioned in a trench of the sheet;

FIG. 8 shows a permanent formwork element, of the type shown in FIG. 5, being manoeuvred into position adjacent another permanent formwork element in the installation of a floor in a building;

FIG. 9 is a detail from FIG. 8;

FIG. 10 is a perspective view of a section of a floor made up of abutting permanent formwork elements of the type shown in FIG. 5;

FIG. 11 is a perspective view of section of a floor in a building under construction, the floor made up of abutting permanent formwork elements of the type shown in FIG. 5;

FIG. 12 is an elevation view of a section of a floor made up of abutting permanent formwork elements of the type shown in FIG. 5;

FIG. 13 is a computer-modelled schematic representation of expected deflections in an embodiment of a composite slab permanent formwork and reinforced concrete element of the present invention;

FIG. 14 is a first schematic cross section view through a composite slab permanent formwork element and the supporting primary falsework beam, of the type shown in FIG. 13 along the line A-A, showing the deflections therein; and

FIG. 15 is a second schematic cross section view through a composite slab permanent formwork element and the supporting primary falsework beam, of the type shown in FIG. 13 along the line B-B, showing the deflections therein.

DESCRIPTION OF EMBODIMENTS

In FIGS. 1 to 4, there is shown a truss 1 comprising an elongate metal strip 10 and an elongate bar 12 that is connected to the elongate metal strip 10.

The elongate metal strip 10 has a first longitudinally extending edge 14 and a second longitudinally extending edge 16. The first longitudinally extending edge 14 and the second longitudinally extending edge 16 are transversely opposed to one another.

The first longitudinally extending edge 14 is in the form of a substantially serrated edge (i.e. a sawtooth-like edge) such that apexes 18 are formed along the first longitudinally extending edge 14.

The serrated edge at the first longitudinally extending edge 14 of the elongate metal strip 10 is formed by cut-out sections at the first longitudinally extending edge 14.

The elongate bar 12 is connected, along its length (i.e. longitudinally), to the elongate metal strip 10 at the apexes 18 of the metal strip 10. This may be done by welding. For example, using spot, resistance, mig, tig or arc welding methods. Typically, the elongate bar 12 is substantially central to the first longitudinally extending edge 14, as best seen in FIG. 4. Consequently, the metal strip 10 and the longitudinal axis of the elongate bar 12 are in the same plane.

Openings 20 are formed between the elongate bar 12 and the serrated edge of the metal strip 10.

The elongate bar 12 may be connected to the elongate metal strip 10 by welding the apexes 18 to the elongate bar 12.

The openings 20 between the elongate bar 12 and the serrated edge of the metal strip 10 may substantially triangular in shape.

The second longitudinally extending edge 16 of the elongate metal strip 10 may be rounded. This is best seen in FIGS. 2 and 4.

The elongate bar 12 may be a concrete reinforcing bar, circular in profile. Concrete reinforcing bars are typically used for reinforcing concrete.

The elongate bar 12 may be made of steel.

To protect against corrosion, the truss 1 may be galvanized and coated in a solid two-part structural polymer. This dual treatment (galvanizing and polymer coating) provides two layers of protection against corrosion.

The truss 1 may be used to make a permanent formwork element.

In FIGS. 5 and 6, there is shown such a pair of permanent formwork elements 50, arranged side by side. Each permanent formwork element 50 comprises a plurality of trusses 1, as herein before described and shown in FIGS. 1 to 4, and at least one sheet 52.

FIG. 7 shows a portion of a permanent formwork element 50. The sheets 52 are normally rectangular in shape. Alternatively, the sheets 52 may be machined into circular, semicircular, triangular or any other desired shape to fit the intended suspended slab shape.

The sheet 52 may be cementitious board material. Typically, a plurality of such sheets 52 are used to make a permanent formwork element 50 because cementitious sheets are generally made in sizes smaller than is required to make a permanent formwork element 50, as such sizes permit easier manufacture and handling of the cementitious sheets.

Trenches, or channels, 54 are formed in the sheets 52. The trenches 54 are best seen in FIG. 7. The trenches 54 extend in the longitudinal direction of the sheets 52 in a substantially parallel arrangement. The trenches 54 are spaced apart at a selected distance.

As previously described herein, typically, a plurality of such sheets 52 are used to make the permanent formwork element 50. For example, the two (longitudinally extending) permanent formwork elements 50 shown in FIG. 5 are each made up of three sheets. The first permanent formwork element 50 (positioned lowermost in FIG. 5) is made up of sheets 52 a, 52 b and 52 c, whilst the second permanent formwork element 50 (positioned uppermost in FIG. 5) is made up of sheets 52 d, 52 e and 52 f. The sheets 52 of a permanent formwork element 50 are arranged such that the ends 24 of adjacent sheets 52 abut. The side edges 22 of adjacent sheets 52 of respective first and second permanent formwork elements 50 also abut. This arrangement of abutting side edges 22 and ends 24 is best seen in FIG. 5. The sheets 52 of each permanent formwork element 50 are arranged in substantially the same plane. Trenches 54 of sheets 52 that have abutting ends 24 are in alignment such that elongated trenches 54 are formed that extend the length of the end abutting sheets 52 that make up that permanent formwork element 50.

Elongate metal strips 10, of the trusses 1, are partly positioned in respective trenches 54. Elongate metal strips 10 extend in respective elongated trenches 54 that are formed by end abutting sheets 52. Thus, for example, in the permanent formwork elements 50 shown in FIG. 5, there are two pairs of three sheets 52 that are arranged in side by side, end abutting arrangement, the first pair being made up of sheets 52 a, 52 b and 52 c (first permanent formwork element 50), and the second pair made up of sheets 52 d, 52 e and 52 f (second permanent formwork element 50). The sheets 52 a, 52 b and 52 c, in end abutting arrangement, have longitudinally aligned trenches 54 creating five elongated trenches 54 in which five respective trusses 1 are positioned. Similarly, sheets 52 d, 52 e and 52 f, in end abutting arrangement, have longitudinally aligned trenches 54 creating five elongated trenches 54 in which five respective trusses 1 are positioned.

The trenches 54 are formed to have a depth such that the metal strips 10 are positioned in the trenches 54 up to substantially the openings 20. That is, the maximum depth to which the metal strips 10 are positioned in the trenches 54 is such that the opening 20 are substantially exposed above the surfaces of the sheets 52. Referring to FIG. 3, the depth to which the metal strips 10 are positioned in the trenches 54 is shown by the distance “D”.

The part of each elongate metal strip 10 that is positioned in trenches 54 may be fixedly positioned therein by adhesive 56, as best seen in FIG. 7. The adhesive 56 is injected into the trenches 54 and the elongate metal strips 10 of the trusses 1 are then inserted into respective longitudinally aligned trenches 54 that extend along end abutting sheets 52. The adhesive 56 may be a high strength structural polymer adhesive. The adhesive 56 is allowed to set forming a strong bond between the elongate metal strip 10, along its entire length, and the sheets 52, such that portions of the elongate metal strips 10 are partly fixed in the trenches 54, and embed therein. This effectively joins all the end-to-end abutting sheets 52 and the trusses 1 into a single large permanent formwork element 50.

The rounded second longitudinally extending edge 16 facilitates insertion of the elongate metal strips 10 into the trenches 54. In addition, the edges of the trenches 54, adjacent to the surface of the sheet 52, are formed with chamfered edges 58 that taper inwardly from the surface of the sheet 52 such that the openings of the trenches 54 are wider than the lower part of the trenches 54. This arrangement may be seen, for example, in FIG. 7. The provision of the chamfered edges 58 also facilitates insertion of the (rounded) second longitudinally extending edges 16 of the elongate metal strips 10 into the trenches 54.

The combined sheets 52 may be of any suitable dimensions. Typically, the maximum size is 2.5 metres by 12 metres. Similarly, the elongate bars 12 may be of any suitable dimensions, as required by the structural requirements of the project. Typically, this is in the range of between 8 mm diameter and 20 mm diameter.

Permanent formwork elements 50 may be transported onsite, where construction of the building is being undertaken. The permanent formwork elements 50 are installed in position, with the trusses 1 uppermost, to construct a floor of the building.

FIG. 8 shows a first permanent formwork element 50 a being manoeuvred into position adjacent a second permanent formwork element 50 b in the installation of a floor in a building. The second permanent formwork element 50 b is already in position, being supported on temporary beams 100 which are themselves supported by upright members 102 known as accrow props. The first permanent formwork element 50 a is suspended by chains 104, from a crane, which can be releasably connected to the trusses 1 by shackles 106 passing through the openings 20 in the trusses 1. The temporary beams 100, upright accrow prop members 102, chains 104 and shackles 106 do not form part of the present invention.

The longitudinally extending side edges 22 of the sheets 52 are provided respectively with a tongue 60 and groove 62, which are shown in FIG. 8, however are best seen in the drawings in FIG. 9, in which they are identified by reference numerals 60 a or 60 b, and 62 a or 62 b to identify the respective permanent formwork element 50 a or 50 b to which they belong. The first permanent formwork element 50 a is manoeuvred into position such that the tongues 60 a of the sheets 52 a of the first permanent formwork element 50 a are received in (i.e. engage with) the grooves 62 b of the sheets 52 b of the second permanent formwork element 50 b. Adhesive can be injected into this groove 62 to structurally bond the tongue 60 and groove together 62.

FIG. 10 shows a section for a floor, identified by reference numeral 64, made up of abutting permanent formwork elements 50 supported on beams 100, and FIG. 11 shows such a section 64 for a floor in a building 110.

After permanent formwork elements 50 are installed, the concrete (or other cementitious filler) is poured over the permanent formwork elements 50 to cover the trusses 1 by the required depth, with the concrete (or other cementitious filler) passing through the openings 20 such that the trusses 1 are embedded in the concrete (or other cementitious filler). Once the concrete (or other cementitious filler) sets, a composite slab is formed. This forms the floor section in the building 110.

The permanent formwork element 50, according to the present invention, may span relatively large spans with deflections in the permanent formwork elements 50 within safe ranges. The sheets 52 and the portions of the elongate metal strips 10 that are fixed in the trenches 54 act together as one flange-like element of the composite slab permanent formwork element slab, whilst the serrated sections of the elongate metal strips 10 acts as a web-like element in the composite slab permanent formwork element, and the elongate bars 12 act as the other flange-like element.

The slab in accordance with the present invention may span relatively larger distances with a relatively thinner sheet 52. The arrangement of the bottom tensile sections of the elongate metal strips 10, such that they are surrounded by a solid two-part structural polymer adhesive on both sides, and the second longitudinally extending edge 16 along the entire lengths of the elongate metal strips 10 within the trenches 54 of the sheets 52, greatly enhances their ability to resist corrosion in situations where corrosive contaminants such as water, salt, carbon dioxide, oxygen and other corrosive contaminants might ingress the suspended slab and cause corrosion particularly in coastal areas and where the elements are exposed such as external balconies, stairs and carparks. The cementitious material of the concrete (or other cementitious filler material) also greatly resists the corrosion of the elongate bar 12 by offering typically 20 mm of cover (which can be varied, as required) in concrete (or other cementitious filler material) to the tensile concrete reinforcing bar 12.

During temporary construction loading and concrete placement in mid-span between primary supports 100, the respective portions of the web-like elements of the trusses 1 that are submerged in the trenches 54, as shown by distance D in FIG. 3, are in tension, as are the bottom faces of the sheets 52. They are both in tension in a composite manner.

After the concrete sets and the falsework (e.g. temporary beams 100 and upright members 102) is taken away, the elongate member 12 of the truss 1 goes into tension in the long term and functions as a tensile reinforcing bar in reinforced concrete. Generally, cementitious materials have relatively poor performance in tension. Therefore, bringing galvanized and polymer coated & protected steel (namely, the trusses 1) into the tensile zone will increase span capacities (FIG. 11).

Due to the superiority of the spanning distance and the structure of the permanent formwork elements 50, the thickness of the sheets 52 may be reduced, for example, sheets 52 that have a thickness of 18 mm (instead of 24 mm) may be used, which is a 33% reduction, and yet it may still be possible to increase the primary spacing of the falsework supports from 1200 mm to 1675 mm which is a 39.5% increase in spacing of the primary members for say a typical 200 mm thick concrete slab.

The bottom of reinforcing bar 12 of truss 1 may be used to easily change the concrete cover distance from typically 20 mm to 60 mm between the surface of the permanent formwork sheet 52 and the bottom of the elongate bar 12 by adjusting the height of the apexes 18. This may be automatically done during the manufacturing process such that the desired height of the apexes 18 is achieved.

The truss 1 may be made with any required sized reinforcing bar (as the elongate bar 12) off a coil typically from N8 to N20 (8 mm diameter to 20 mm diameter) which may result in typically no extra main directional reinforcing bar being required in the direction of the main span for a one-way spanning slab as the reinforcing bar in the composite truss 1 is the main elongate bar 12.

Since the truss 1 is spaced correctly during the manufacturing process, sometimes no distribution reinforcing steel in the opposite direction is required. Similarly, tensile steel is required only over supports under the slab. Steel fibres are used to control cracking in the concrete. This results in a much less labour intensive installation or less reinforcing steel and reduces costs significantly.

The trusses 1 be placed at any desired spacing in the sheets 52 to suit the permanent formwork element slab spans because of the way the trenches 54 are milled into sheets 52. Typically, this ranges from 150 mm to 300 mm spacing but may be smaller or greater depending upon the specific engineering requirements for the installation.

Another feature of the present invention described herein is the ability of the individual sheets 52 to be tongue and grooved, or ship lap joint and adhered together using adhesive and or mechanical fasteners to form one large permanent formwork element 50. This allows the whole permanent formwork element 50 to expand and contract as one which allows expansion joints to be placed where required in obscure locations that do not affect the architectural aesthetics of the building. This also allows the soffit of the element to be flushed (taped and jointed) at the joint instead of a glass reinforced skim coat being applied to the whole ceiling soffit. Further, as shown in FIG. 12, a 45-degree recess connection 66 may be provided to allow the sheets 52 to freely expand and contract at the interface with a wall 108 which relieves stress in other adhered joints in the centre of the ceiling, helping to reduce the possibility of cracking in the sheets 52. The recess connection 66 is formed by providing a mating 45-degree chamfer on the longitudinal edge of the sheets 52 and the wall 108.

Another feature of the present invention described herein is the ability of the composite permanent formwork elements 50 to be arranged in a pre-calculated negative camber along the direction of the main span for a one way spanning slab. This is achieved by arranging the upright members 102 at different pre-calculated heights (FIG. 11) so that when the composite permanent formwork elements 50 are laid on them, the self-weight of the permanent formwork arches the permanent formwork element 50 into negative camber. The amount of negative camber is calculated prior by calculating the deflection for the given span in either a one way or two way spanning slab. This has the net effect of the slab span for any given spacing arrangement of trusses 1 and thickness being limited by the ultimate limit state design of the element as opposed to long term deflections. It may be possible to increase the span for any given slab thickness and arrangement of trusses 1 by approximately 30% using this methodology.

The wet concrete (or other cementitious filler material) to be laid on the permanent formwork elements 50 may have local falls to suit the expected deflection profile and to keep the concrete thickness reasonably constant. It also ensures that the top surface of the concrete does not overtly sag. Alternatively, the concrete can be laid flat as traditionally. After the propping and back propping is taken away and the suspended slab is loaded, the concrete surface on top will sag a little. Floor levelling compound can then be used to fill the valleys resulting in a concrete slab that is flat on top and flat on the bottom with fully tensioned elongate bars 12. Typically, around 85% of a slab's deflection is from the self-imposed weight. By cambering the soffit of the slab negatively we can arrest around 85% of deflection.

The simple one-way spanning slab deflection profile can be approximated using a simple slab deflection profile. However, when a designer wishes to design a 2-way spanning slab a Finite Element Analysis software is typically employed to economize concrete thickness and reinforcing bar rates. The reinforcing bar rate refers to the total cross-sectional area of the reinforcing bars in a slab relative to the total cross-sectional area of the slab. Depending on other elements around the slab, the actual expected deflections of the slab may vary greatly. These various deflections can be predicted using Finite Element Analysis computer programs. These are complex mathematical calculations of the slab element broken into thousands of smaller parts. An example of the output is shown in FIG. 13.

As can be seen in FIG. 13 from the variations in shading in the slabs, the actual deflections can vary significantly within the 12 slabs shown being analysed. In this example, the multi-spans on the three slabs in the central row R allow for less deflections (lighter shading) than, for example, the corner slabs C (darker shading), which are constrained on only two sides. In contrast, the two central slabs in the central row R are constrained on four sides and the two end slabs in the central row R are constrained on three sides. Traditionally, the way to deal with excessive deflections is to increase the concrete depth and/or increase the cross-sectional area of reinforcing bars.

However, according to the present invention, the manner in which excessive deflections are dealt with is to profile the actual expected deflections of the slab where the primary formwork support will be located. By taking a cross section through the slab model, which shows the expected deflections at the planned locations of the primary supports, it is possible to identify the amount by which the slab will deflect. That calculated cross-section data is then used in a computer program (for example, a CAD/CAM program) to machine timber strips such that they have profiles that are symmetrically opposite, or mirrored, to the calculated deflection profile. In this way, the profiles of the timber strips are mirror images of the calculated deflection profile of the slab, such that low regions in the calculated deflection profile of the slab are correspondingly raised regions in the profile of the timber strips; and, conversely, raised regions in the calculated deflection profile of the slab are correspondingly low regions in the profile of the timber strips.

The timber strips are then fixed (e.g. by nailing) onto the top of the primary supports in a pre-planned sequence. The primary support tops are kept at a constant level. The net result is a topography of supports that is exactly opposite to the calculated expected deflections of the slab. The permanent formwork elements 50 are then craned onto this support topography. The connecting reinforcing bar is fixed in position. The fibre reinforced concrete (or other cementitious filler material) is poured onto the suspended deck. It is allowed to set, i.e. harden and strengthen. The falsework, i.e. the supports, below is removed. The slab begins to deflect downwards. As the reinforcing bar begins to come under tension, the deflection is stopped as the reinforcing bar strain hardens. Because the predicted deflection has been countered using opposite topography, the slab is expected to complete its initial deflection to a substantially flat position. This should also reduce the amount of cracking in the tensile zone of the concrete. Since there is an arch profile in the overall slab, the concrete at the bottom of the slab goes into compression instead of tension as the self-weight of the slab element takes effect. This provides a more advanced application of the negative camber application.

FIGS. 14 and 15 show schematic cross-sections along the lines A-A and B-B, respectively, though the slab structure in FIG. 13, showing deflections therein. The deflections have been exaggerated for the purposes of pictorial representation. The calculated (i.e. theoretical) deflection line is shown by reference numeral 120. The timber strips 122 are profiled to arrest deflection of the slab, as described in the preceding paragraph herein. Trusses 1 and permanent formwork elements 50 are shown in position above the timber strips 122. The temporary beams 100 and upright members 102 are also shown.

Whilst one or more preferred embodiments of the present invention have been herein before described, the scope of the present invention is not limited to those specific embodiments, and may be embodied in other ways, as will be apparent to a skilled addressee.

Modifications and variations such as would be apparent to a person skilled in the art are deemed to be within the scope of the present invention. 

What is claimed is:
 1. A truss comprising an elongate metal strip having a first longitudinally extending edge and a second longitudinally extending edge, cut-out sections provided at the first longitudinally extending edge and apexes are formed along the first longitudinally extending edge, and an elongate bar connected to the elongate metal strip at the apexes of the elongate metal strip, such that openings are formed between the elongate bar and the first longitudinally extending edge, having the cut-out sections, of the elongate metal strip, and wherein the elongate bar is connected along its length to the metal strip such that the elongate bar is located substantially central to the first longitudinally extending edge of the elongate metal strip.
 2. A truss according to claim 1, wherein the metal strip and the longitudinal axis of the elongate bar are in the same plane.
 3. A truss according to claim 1, wherein the first longitudinally extending edge is a serrated edge formed by the cut-out sections.
 4. A truss according to claim 1, wherein the elongate bar is connected to the metal strip by welding the apexes to the elongate bar.
 5. A truss according to claim 1, wherein the openings are substantially triangular in shape.
 6. A truss according to claim 1, wherein the second longitudinally extending edge is rounded.
 7. A permanent formwork element comprising a plurality of trusses, according to any one of the preceding claims, arranged in a substantially parallel arrangement, and at least one sheet of material having trenches formed therein, the trenches being in a substantially parallel arrangement, wherein the elongate metal strips are partly fixedly positioned in respective trenches such that second longitudinally extending edges of the elongate metal strips are positioned in the trenches.
 8. A permanent formwork element according to claim 7, wherein the elongate metal strips of the plurality of trusses are partly fixedly positioned in respective trenches such that the openings are substantially exposed above the surface of the sheet.
 9. A permanent formwork element according to claim 7, wherein the elongate metal strips of the plurality of trusses are partly fixedly positioned in respective trenches by adhesive.
 10. A permanent formwork element according claim 7, wherein the edges of the trenches adjacent to the surface of the sheet are chamfered.
 11. A permanent formwork element according to claim 7, wherein the sheet is made of cementitious based material.
 12. A method of making a permanent formwork element, according to claim 7, comprising providing at least one sheet of material having trenches formed therein, the trenches being in a substantially parallel arrangement, inserting the second longitudinally extending edges of the elongate metal strips into respective trenches to position them in the trenches, and fixing the elongate metal strips in the trenches.
 13. A method of making a permanent formwork element according to claim 12, wherein fixing the elongate metal strips in the trenches comprises using adhesive to fix the elongate metal strips.
 14. A method of making a slab comprising calculating the expected deflections in a slab to comprise a permanent formwork element, according to claim 7, and cementitious filler material to cover the permanent formwork element, machining timber strips to have profiles that are mirror images of the profiles of the calculated expected deflections, fixing the timber strips to supports, positioning the permanent formwork element on the supports, pouring cementitious material over the permanent formwork element and allowing the cementitious material to set, and removing the supports.
 15. A method of making a slab according to claim 14, wherein the slab is a one-way spanning slab.
 16. A method of making a slab according to claim 14, wherein the slab is a two-way spanning slab. 